Use Caution when Fertilizing with Raw Horse Manure

For home gardeners, spring is a busy time of year and there’s never a tomato with more flavor than one grown to full ripeness on the vine. But there are also many safety precautions to follow to prevent contamination of fruits and vegetables with pathogens that cause serious foodborne illness.

Michele Jay-Russell, DVM, MPVM, PhD, Dipl. ACVPM, a veterinarian and research microbiologist at the Western Institute for Food Safety and Security and program manager of the Western Center for Food Safety, recently co-authored a study that highlights the need to be aware of the hazards associated with using raw animal manure to fertilize home gardens. The study will be published in an upcoming issue of the journal Zoonoses and Public Health.

The basis for the study began in July of 2010 when a Shire mare from a rural Northern California farm was brought to the University of California, Davis, William R. Pritchard Veterinary Medical Teaching Hospital for treatment of colic. Following protocol, the veterinarians on call screened the horse for Salmonella to avoid infecting other horses during hospitalization. She tested positive and after successful treatment for colic, went home. Her owners then notified the veterinarians that some of their other draft horses were sick as well—all eight were tested and six came back positive for the same Salmonella Oranienburg strain, including the mare that still had the infection.

Jay-Russell heard about the case from her colleague John Madigan, DVM, MS, Dipl. ACVIM, professor of medicine and epidemiology at the school. The farm’s owners invited Jay-Russell and Madigan to the farm to see if they could uncover the source of the Salmonella infection. They sampled water from horse troughs, manure storage piles, wild turkey feces, and soil from the family’s edible home garden, where raw horse manure had been used as fertilizer. Each of those locations had a percentage of positive samples over the sampling period from August 2010 to March 2011.

“We showed the owners how to continue collecting samples and provided them with a Fed Ex number to ship them to UC Davis,” Jay-Russell said. “During that whole time, the garden soil kept coming back positive, which showed that this strain of Salmonella could persist for months.”

While the researchers couldn’t be completely certain about the original source of Salmonella on the farm, they suspect that a recent surge in the wild turkey population on the property introduced the bacteria to the horses by pooping in the horse corrals and in the water troughs. They speculated that the wild turkeys brought the Salmonella onto the property, although they couldn’t rule out the possibility that the birds were exposed on the farm or to other potential sources of Salmonella.

“What is clearer is that the raw horse manure applied as fertilizer was the most likely source of garden soil contamination,” Jay-Russell explained. “We suspect that the damp climate in Mendocino County may have contributed to the longevity of this bacterium in the soil long after the owners stopped applying the horse manure to the garden. Fortunately, the owners didn’t get sick, but our investigation showed the potential for widespread dissemination of Salmonella in a farm environment following equine infection.”

To promote safe gardening practices, Jay-Russell has teamed with Trevor Suslow, PhD, a Cooperative Extension food safety specialist in the Department of Plant Sciences, to speak to groups of small farmers around the state about best practices. They also use a brochure, “Food Safety Tips for Your Edible Home Garden,” that includes information about safe uses of animal manure and ways to minimize animal fecal contamination.

“It’s good to let people know about the risks and to correct misinformation about ways to treat the compost pile before using it in the garden,” Jay-Russell said. “The biggest take home message from this experience is to be very careful about using manure from sick horses and to be cautious about offers of free manure—you don’t know what’s in there. Commercial compost should be bought from a reputable source.”

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